There are many fields in which measurements of electric or magnetic fields are utilized for resolving the direction of arrival of electro-magnetic (EM) wavefronts. These may be used, for example, in a system for determining spatial geo-location of emitters applicable in search or rescue applications.
There are also applications in which mutual co-located measurement of both the electric and magnetic fields are required.
As is well known, an electric dipole of length l is considered a small dipole (in terms of wavelength) when l<<λ, in that range, and approximation of the current distribution is uniform, which allows calculation of the radiated fields. In the far-field region (kr>>l) E and H-fields radiated by a small dipole aligned with the {circumflex over (z)} axis comply with the following equation (1):
                                                                                                                                    E                      θ                                        =                                          j                      ⁢                                                                                          ⁢                      η                      ⁢                                                                                          ⁢                                                                                                    kI                            0                                                    ⁢                          l                          ⁢                                                                                                          ⁢                                                      ⅇ                                                                                          -                                j                                                            ⁢                                                                                                                          ⁢                              k                              ⁢                                                                                                                          ⁢                              r                                                                                                                                8                          ⁢                          π                          ⁢                                                                                                          ⁢                          r                                                                    ⁢                                              sin                        ⁡                                                  (                          θ                          )                                                                                                                                                                                                            E                      r                                        =                                                                  E                        ϕ                                            =                                                                        H                          r                                                =                                                                              H                            θ                                                    =                          0                                                                                                                                                                                                            H                      ϕ                                        =                                          j                      ⁢                                                                                                                                                        ⁢                                                                                    kI                              0                                                        ⁢                            l                            ⁢                                                                                                                  ⁢                                                          ⅇ                                                                                                -                                  j                                                                ⁢                                                                                                                                  ⁢                                kr                                                                                                                                                              8                          ⁢                          π                          ⁢                                                                                                          ⁢                          r                                                                    ⁢                      sin                      ⁢                                                                                          ⁢                                              (                        θ                        )                                                                                                                  }                    ⁢          kr                >>        1                            (        1        )            
where I0 is a constant. Note that for sensing an electric field in the Z axis, Eθ is the electric field value of interest.
Turning now to magnetic fields, for the small current loop (in terms of wavelength, (l<<kr) the current distribution is given by Iφ=I0 where I0 is a constant.
The electric and magnetic fields in a far-field (kr>>l) region for a small magnetic loop are given by:
                                                                                                                                    H                      θ                                        =                                                                  -                                                                                                                                            (                                ka                                )                                                            2                                                        ⁢                                                          I                              0                                                        ⁢                                                          ⅇ                                                                                                -                                  j                                                                ⁢                                                                                                                                  ⁢                                kr                                                                                                                                          4                            ⁢                            r                                                                                              ⁢                                              sin                        ⁡                                                  (                          θ                          )                                                                                                                                                                                                            E                      ϕ                                        =                                          η                      ⁢                                                                                          ⁢                                                                                                                                  (                              ka                              )                                                        2                                                    ⁢                                                      I                            0                                                    ⁢                                                      ⅇ                                                                                          -                                j                                                            ⁢                                                                                                                          ⁢                              kr                                                                                                                                4                          ⁢                          r                                                                    ⁢                                              sin                        ⁡                                                  (                          θ                          )                                                                                                                                                                                                            E                      r                                        =                                                                  E                        θ                                            =                                                                        H                          r                                                =                                                                              H                            ϕ                                                    =                          0                                                                                                                                          }                    ⁢          kr                >>        1                            (        2        )            
Note that for sensing a magnetic field in the Z axis (coinciding with the specified Z axis direction of Eθ) Hθ is the magnetic field value of interest.
As is well known, the electric dipole responds to an electric field aligned with a dipole axis while the current loop responds to a magnetic field perpendicular to its containing plane.
When two or more antennae are in the vicinity of each other, whether one and/or more are transmitting or receiving, some of the energy that is primarily intended for one antenna ends up at the other antenna, giving rise to mutual coupling. The amount depends, amongst others, on:
a. radiation characteristics of each antenna
b. relative distance between the antennas
c. relative orientation of the antennas
d. port loading (impedance of the load)
Mutual coupling may be distractive to the ability to sense specific field components by an antenna element in the vicinity of another antenna element targeting other field components, since part of the energy related to undesired components may leak through mutual coupling and distort the desired component sense.
There are different systems for sensing electric fields and magnetic fields. Systems include the R&S HM-020 shown for instance in FIG. 1, that is composed of three current loops (101 to 103), disposed perpendicularly to each other and which are capable of sensing a magnetic field (only) in three perpendicular directions, respectively. The device of Bergman et al. as shown in FIG. 2 is composed of three dipoles (201 to 203) that are disposed perpendicularly to each other and are capable of sensing an electric field (only) in three perpendicular directions, respectively.
Turning now to known co-located systems, attention is drawn to FIG. 3, illustrating schematically a co-located device for measuring magnetic and electric fields, in accordance with the prior art. As shown, the system includes three distinct co-axial arrangements where ring 301 and coaxial dipole 302, disposed perpendicularly thereto, are capable of sensing a respective magnetic field and electric field projections, in the Z direction. Similarly, ring 305 and coaxial dipole 304, disposed perpendicularly thereto, are capable of sensing a respective magnetic field and electric field projections, in the Y direction. And, ring 303 and coaxial dipole 306, disposed perpendicularly thereto, are capable of sensing a respective magnetic field and electric field projections, in the X direction.
The measurements of the projections of the electric and magnetic fields along the Z, Y and X axes can further be utilized for the calculation of the properties (including propagation direction) of the electro-magnetic (EM) wavefront, by applying the Poynting Theorem. According to the latter, an electromagnetic wavefront has a unique relation between the electric and magnetic field components to its direction of propagation by
                              S          _                =                              1            2                    ⁢                      E            _                    ×                                    H              *                        _                                              (        1        )            
where Ē and H are phasors. The magnitudes |Ē| and | H| are peak values, and therefore the RMS values are |Ē|/√{square root over (2)} and | H|/√{square root over (2)}, respectively. The S vector gives the direction and the RMS value of the complex power flux density. By knowing the field components, the direction of propagation is uniquely determined. Note that the direction of arrival of such a wavefront would be determined to be opposite to the direction of propagation. Note that generally the S direction may be represented in a given coordinates system with origin located at the measurement point by an azimuth angle (φ) and an elevation angle (θ) at this coordinates system.
There are a number of limitations in the system depicted in FIG. 3, including:                relatively high mutual coupling        cumbersome feed circuitry (6 feeds)        more sensitivity to electric field than a corresponding proportional magnetic field.        
Turning now to FIG. 4A, it illustrates a slot cut in a generalized structure, according to the prior art. As shown, a slot antenna 40 is produced by creating a thin slot of length h 41 in a conducting metal sheet and feeding it into the center of the slot (not shown). The radiation pattern of a slot antenna is identical to that of the electric dipole of the same length, except that orientations of the E and H fields are interchanged. This means that the magnetic dipole can be replaced by a slot.
Turning now to FIG. 4B, it illustrates schematically a co-located device for measuring magnetic and electric fields (utilizing the specified concept of a slotted element), in accordance with the prior art. The device had been first suggested by Smith et al. for polarization synthesis, however, this prior construction requires tight dimensions of the elements in terms of wavelength. In other words, the longitudinal dimension of the device 401 should be substantially identical to the wavelength λ of the sensed fields, and therefore must be customized to each specific wavelength λ of the sensed field. This constraint stems from the fact that the original form of feeding circuitry to slots 402 affects the electric dipole (shorts it) unless the electric dipole's arms 403 are half wavelength (λ) each. Note that this shortcoming renders the apparatus practically infeasible for sensing fields at rather low frequency (2-30 MHz) as the latter imposes a very large apparatus size. Considering also the apparatus described with reference to FIG. 3, there is, thus, a need in the art to provide for a new co-located apparatus for resolving the direction of arrival of an electro-magnetic (EM) wavefront.